Cation Exchange Capacity of Soils - Laboratory 8 | NRES 201, Lab Reports of Earth Sciences

Download Cation Exchange Capacity of Soils - Laboratory 8 | NRES 201 and more Lab Reports Earth Sciences in PDF only on Docsity! Introductory Soils Lab 8 Cation Exchange Capacity NRES 201 LABORATORY 8. CATION EXCHANGE CAPACITY OF SOILS 8.1 GENERAL CONCEPTS. Cation exchange is the reversible, low-energy transfer of ions between solid and liquid phases. Cation exchange affects many soil processes: 1. Weathering of soil minerals. 2. Nutrient absorption by plants. 3. Leaching of electrolytes. 4. Buffering of soil pH. Cation exchange is the result of the neutralization of the negative charge on soil colloids by oppositely charged cations. The cations are held to the colloidal surface by columbic attraction with van der Waal forces and induction forces increasing the strength of bonding of some types of cations. Along with the attraction and subsequent concentration of cations which are oppositely charged to the colloidal surface, is the repulsion of anions which are negatively charged like the colloidal surface. 8.2 CONCEPTUAL MODEL OF THE CATION EXCHANGE PROCESS. Cations are not rigidly held on the colloidal surface, but because of their thermal energies have some degree of motion on and away from the surface, such that a hemisphere of motion is defined for each particular combination of ion and colloidal surface. Consider two cases, the first case, a tightly held and hence, nonexchangeable or fixed cation and the second case, a less tightly held exchangeable cation. Page 1 of 8 Introductory Soils Lab 8 Cation Exchange Capacity NRES 201 What is cation exchange? Cation exchange occurs (Fig. 2a) when ions (⊗) in the bulk solution move into the hemisphere of motion of a cation on the surface (ּס) at a point in time when the exchangeable cation is far from the surface. The ion initially in solution becomes trapped on the surface by the negative charge, and the ion initially on the surface moves into the soil solution. If the surface ion is close to the charged colloidal surface when the solution ion randomly moves into the hemisphere of motion, ion exchange does not occur (Fig. 2b), and the ion returns to the solution. The motion of the ions in the bulk solution is due to their thermal energies. Many factors affect the distribution of cations between the soil solution and the colloidal surface. An intuitive feel for the factors affecting cation exchange can be developed using the conceptual model. 1. As the concentration of a particular cation in the bulk solution increases, the probability of that type of cation penetrating the hemisphere of motion of a surface cation at a time when the surface cation is at a distance from the surface also increases. Hence, as the concentration of a cation in solution increases, there is a corresponding increase in the amount of that cation on the colloidal surface (exchange site). 2. The effect of ion valence can also be illustrated. As the valence of an exchangeable cation increases, so does the affinity of the cation for the surface resulting in a smaller hemisphere of motion. Hence, as valence increases the exchangeability of the ion decreases and the cation’s concentration on the colloidal surface increases relative to cations of lower valence. Other factors such as the hydrated size of the cation and the density of charge on the colloidal surface also affect the degree of attraction of the cation to the colloidal surface. The hydrated size of the cation determines how close the cation can approach the negatively Page 2 of 8 Introductory Soils Lab 8 Cation Exchange Capacity NRES 201 3. Heat the soil with potassium chloride solution under alkaline conditions, so as to: (a) displace exchangeable ammonium with potassium, (b) liberate the ammonium displaced as gaseous ammonia, and (c) collect the ammonia in boric acid-indicator solution. 4. Determine the CEC by titrating the boric acid solution with standard sulfuric acid. Laboratory procedure. 1. Determination of CEC. a. Place a 10-ml syringe into one of the stopcocks on the extraction manifold, and insert a stainless-steel frit into the syringe. b. Weigh 0.50 gram of air-dry soil into the syringe, taking care not to jar the syringe and thus ensure that the soil remains on top of the frit. c. With the manifold under vacuum, open the stopcock below your syringe, and introduce 5 ml of 1 N ammonium acetate solution against the wall of the syringe using a 5-ml pipette. Repeat the latter step twice, so as to leach the soil sample with a total of 15 ml of ammonium acetate. d. When the ammonium acetate solution has drained completely, maintain vacuum while filling the syringe with approximately 10 ml of isopropyl alcohol from a graduated cylinder. Repeat twice for a total of 30 ml of isopropyl alcohol. e. Allow the isopropyl alcohol to drain completely, and maintain vacuum for at least 5 minutes to thoroughly dry the soil sample. f. After closing the supporting stopcock to discontinue vacuum, remove your syringe and invert it over a 1-pint wide-mouth Mason jar, thereby transferring the soil sample and the frit. g. Prepare a Mason jar lid for use by strapping a petri dish into the cable tie, and then dispense 5 ml of boric acid-indicator solution into the dish. g. Add 10 ml of 4 M KCl to the jar, followed by 1 scoop (approx. 0.2 g) of MgO powder. h. Immediately seal the jar by attaching the lid with a screw band. i. Transfer the sealed jar to a griddle previously adjusted to provide a surface temperature of 45-50ºC. j. After 1.5 hours, remove the jar from the griddle and carefully extract the petri dish from the jar without spilling its contents. k. If the original reddish color of the boric acid solution persists after diffusion, add 5 ml of Page 5 of 8 Introductory Soils Lab 8 Cation Exchange Capacity NRES 201 deionized water to the petri dish. l. While swirling the petri dish, carefully dispense 0.02 N sulfuric acid from a buret until the boric acid solution has just assumed its original reddish color. Record the volume and exact normality of the sulfuric acid. Data – CEC experiment. Soil 1 Soil 2 Soil 3 Milliliters of sulfuric acid used in titration (V) ________ ________ ________ Exact normality of sulfuric acid (N) ________ Calculations – CEC experiment. CEC (meq/100 g or cmolc/kg) = NV × 100 g/0.50 g = 200NV 2. Flocculation experiment. a. Pipette 5 ml of the dispersed clay provided by the T.A. into 5 test tubes. b. Add 1 ml of the following salt solutions to different 5-ml samples in the test tubes. 1 N KCl, 1 N NaCl, 1 N CaC12, 1 N AlCl3, and distilled water. c. Label each tube to identify the salt solutions added. d. Swirl the tubes to mix the salt solutions with the suspension. e. Observe and record the rate of flocculation and the size of the floccules. Rank the cations as to their effectiveness in flocculating the clay suspensions. Discuss the reasons for the observed differences. Data – flocculation experiment. 1 N KCl Observations 1 N NaCl Observations 1 N CaC12 Observations 1 N AlCl3 Observations Distilled water Observations Page 6 of 8 Introductory Soils Lab 8 Cation Exchange Capacity NRES 201 3. Physical properties of clays: a. Add two grams of kaolinite to a plastic beaker. b. Fill a 100-ml graduated cylinder with tap water. c. Add water slowly from the cylinder to the clay in the beaker until a smooth creamy mixture is obtained. d. Record the volume of water used. e. Repeat the above procedure with two grams of montmorillonite. Discuss the reasons for the different amounts of water needed to produce the smooth creamy mixtures (pastes). Data physical properties of clays Weight of kaolinite __________ ml water to make paste __________ Weight of montmorillonite __________ ml water to make paste __________ 4. Write up the experiments, including a brief introduction, the data, calculations and interpretations of the results. 5. Answer the following questions and include them with your laboratory write-up. a. NH4+ is used to replace the original exchangeable ions in this experiment. Is it possible to use other cations? b. What factors determine the amount and type of cations on the exchange complex? c. Why are exchangeable cations not held lightly to the colloidal surface? d. What effect does soil CEC have on the relative loss of cations and anions by leaching? e. Discuss the difference between an “exchangeable” and a “fixed” cation. f. Why do only certain types of clays “fix” K+ and/or NH4+? g. How would you modify the CEC experiment to determine the nature and amount of exchangeable cations originally present in the soil? h. What is the purpose of using alcohol instead of water to remove the excess NH4+ ions from the soil? Page 7 of 8